Missing cosmic lithium shows up in a neighboring galaxy

Study solves one problem, creates another: is lithium made other ways?

The Small Magellanic Cloud (SMC), one of the Milky Way's satellite galaxies. Because the SMC contains relatively fewer metals (elements heavier than helium), astronomers were able to measure the amount of lithium in the galaxy more precisely than before.

An early success of the Big Bang model was the prediction of the primordial amounts of hydrogen and helium, a product of the first formation of atoms, known as the Big Bang nucleosynthesis (BBN). However, BBN predicts roughly four times more lithium than we've seen in the atmospheres of stars.

Lithium is far harder to create in stars than elements such as carbon, oxygen, and the like. Therefore, most of the lithium present in the Universe today was created in approximately the first three minutes after the Big Bang, with the balance being made by high-energy cosmic rays. (That's right: your gadgets run on a remnant of the Big Bang.) Thus, the missing lithium has been a minor problem for BBN, leading to some speculation about where it might be found.

Cosmologists may be able to rest a little easier: new observations of interstellar gas in the Small Magellanic Cloud (SMC) have measured a lithium abundance consistent with BBN. The SMC (a satellite galaxy of the Milky Way) contains a lot fewer heavier nuclei than the Milky Way, meaning its chemical composition is closer to that of the early Universe.

In addition to observations, astronomers J. Christopher Howk, Nicolas Lehner, Brian D. Fields, and Grant J. Mathews calculated the amount of lithium that should be present in interstellar gas. Since their observational results are so close the predictions of BBN, they actually lead to new problems: explaining why stars have less lithium than expected, and where the lithium produced via cosmic rays could have gone.

Most atoms in the Universe are hydrogen or helium; any heavier elements, known perversely by astronomers as metals, were almost all made via nuclear fusion in stars or supernovae. (The Big Bang, hot as it was, couldn't sustain fusion during inflation, so very little fusion happened after about 5 minutes.) Lithium is awkwardly in between the light elements and metals. It tends to be destroyed in stellar cores, so the BBN was responsible for most of the lithium present in the Universe today, according to widely accepted models.

Figuring out how much lithium was formed during the BBN is a relatively simple calculation, requiring just the ratio of ordinary matter (as opposed to dark matter) to photons in the early Universe. That number is very well constrained by the cosmic microwave background (CMB). Modern lithium abundances are calculated using a combination of BBN and cosmic ray levels (high-energy protons can smash into helium atoms, creating lithium), and a tiny amount lost through destruction by stars.

To test whether these calculations were right, previous observations concentrated on the outer layers and atmospheres of old stars, which are low in metal content. However, these observations found far too little lithium, meaning either BBN required correction from physics beyond the Standard Model, or some process affected the amount of lithium in stars.

If the latter is true, looking at something other than stars should reveal the missing lithium. Which is why the researchers were looking at the gas in the SMC.

Interstellar gas again is mostly hydrogen and helium, but in environments like the Milky Way, generations of stars have enriched it with metals. The SMC, on the other hand, is metal-poor, so the chemical makeup of the gas it contains should more closely reflect the primordial abundances. Another advantage to viewing the SMC is its proximity—since it orbits the Milky Way, it's relatively easy to observe individual stars.

The researchers looked at a particular supergiant star and measured how its light was absorbed by intervening gas along the line of sight. This gave them the lithium abundance, which isn't the absolute number of atoms present, but the relative number compared to hydrogen or other elements. In this case, the authors calculated the expected fraction of lithium nuclei compared with potassium and iron.

Combining the calculations with observational data, the researchers found the best fit was the predictions of BBN alone, without contributions from stars or cosmic rays—a somewhat puzzling result. They postulate that perhaps a smaller amount of primordial lithium might exist due to deviations from basic BBN, meaning the cosmic ray production could raise it to the levels we see, but that introduces a kind of fine-tuning problem, where we arbitrarily tweak models to get the results we see.

The hope now is that further observations may be able to constrain alternatives to the simplest form of BBN (which might involve extensions to the Standard Model), or see what other mechanisms might exist to produce lithium.